Ship bottoms, buoys, fishing nets (e.g., nets for cultivating young yellowtail, scallops, etc., and stationary nets for catching salmon), and other structures submerged in seawater such as marine antifouling sheets and cooling water intake or discharge pipes are infested with organisms such as barnacles, tube worms and algae that attach to the surfaces of these structures and cause various troubles. It is routine practice to prevent the attachment of these marine fouling organisms by coating the surfaces of the aforementioned items with antifouling paints. Antifouling paints are roughly divided into the following two classes.
(A) An antifouling paint that employs antifoulants such as organotin copolymers and cuprous oxide that are capable of preventing the attachment of fouling organisms and have low solubility in seawater. Paints that employ organotin compounds as antifoulants are shown in Japanese Patent Publication Nos. 21426/65, 9579/69, 13392/71, 20491/74, 11647/76 and 48170/77.
(B) An antifouling paint that does not employ any antifoulants and which will not dissolve in seawater; instead, it uses silicone rubbers that cure by the action of a catalyst or moisture to form a crosslinked film For instance, an antifouling paint that uses a curable silicone rubber as a coating agent is shown in Japanese Patent Publication No. 35974/78. An antifouling paint that uses a mixture of a silicone oil and an oligomer-like silicone rubber having a terminal hydroxyl group is shown in Japanese Patent Application (OPI) No. 96830/76 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). A mixture of a curable silicone rubber and a flowable organic compound that does not contain a metal or silicon is shown in Japanese Patent Application (OPI) No. 79980/78. A paint that serves to prevent the attachment of fouling marine organisms is also shown in Japanese Patent Publication No. 3433/85 and this paint is composed of a mixture of an oligomer-like low temperature curing silicone rubber (such as those available from Shin-Etsu Chemical Co., Ltd. under the trade names of "KE 45 TS" and "KE 44 RTV") and liquid paraffin or petrolatum.
These known antifouling paints exhibit characteristic performance depending upon their type and have been used in applications that suit specific object. However, these paints have the following problems to be solved.
The antifouling paints of class (A) are further divided into two subclasses. In one subclass of such antifouling paints, the film-forming resin does not dissolve in seawater and only the antifoulant dissolves in seawater to prevent the attachment of marine organisms. The paint films formed from this class of antifouling paints exhibit the intended effect during the initial period of application but after the antifoulant on the surface of the film is lost as a result of its dissolution in seawater, the antifoulant in the interior of the film will gradually dissolve. However, the dissolution rate of the antifoulant decreases as the depth of the area of the paint film in which the antifoulant is present increases, and the antifouling effect of the film will diminish with time.
In the second subclass of antifouling paints of class (A), both the antifoulant and the film-forming resin dissolve in seawater. The antifouling effect is achieved solely by the antifoulant or by a combination of the antifoulant and the resin component (e.g., an organotin copolymer) and, in either case, the surface of the paint film dissolves in seawater, continuously providing the antifouling paint film with an active surface. Therefore, the film formed from this type of antifouling paints is capable of maintaining the desired antifouling effect over a longer period than the aforementioned first subclass of paints (A). However, the effect of this type of antifouling paints is not completely satisfactory because the paint film they form is consumed fairly rapidly.
Antifouling paints of class (B) are designed to prevent the attachment of marine organisms by making use of the slipping property (low surface energy) of the silicone rubber coating. These antifouling paints have the advantage that they do not contain any component that will dissolve in seawater to cause its pollution as do antifouling paints of class (A). However, the mechanism of film formation from these paints involves the crosslinking of silicone rubbers after paint application and presents the following problems.
The first problem is associated with the curing of the film of the applied paint. For instance, when an antifouling paint of the type described in Japanese Patent Publication No. 3433/85 that employs a low temperature curing oligomer like silicone rubber that cures by the action of moisture in air to form a paint film is applied to a substrate, the crosslinking agent incorporated to control the curing condensation reaction of the silicone rubber is activated by the moisture or temperature of air to cause premature curing of the surface of the paint film. This retards the curing of the interior portion of the paint film to produce an insufficiently cured film which is most likely to blister or separate from the substrate. Furthermore, the slow penetration of moisture into the bulk of the film prolongs the time required to achieve its complete curing.
If the antifouling paint of the type described above is applied in a hot and humid atmosphere, the hydrolysis of the crosslinking agent predominates over the crosslinking reaction and the resulting paint film does not have a sufficient crosslinking density to provide satisfactory properties.
In a dry climate, the amount of aerial moisture is too small to cause hydrolysis of the crosslinking agent and the applied paint will cure very slowly. In order to avoid this problem, catalysts such as tin compounds and platinum are sometimes used as curing accelerators but their effectiveness is limited in cold climates.
The second problem concerns topcoating. In the usual case, the solvent in a paint for topcoating slightly softens the surface of the undercoat to ensure good intercoat bonding. However, in the application of the antifouling paint under consideration, the silicone rubber in the first applied coating cures to such an extent that the solvent in a paint for topcoating is not capable of softening the surface of the silicone rubber to provide satisfactory intercoat bonding.
The third problem is related to pot life. In practice, the length of coating operations may be extended beyond the scheduled period of time if the item to be treated is large in size or has a complex structure. In addition, the operation may be interrupted by unexpected rainfall or increase in the humidity of the air. In these cases, the paint which has been stirred in an open container must be left to stand until the surface of the substrate becomes sufficiently dry to warrant continued application of paint. In view of these possibilities, antifouling paints having short pot lives present great inconvenience in coating operations.
The fourth problem is associated with storage stability. Antifouling paints, after being prepared, are stored until use and the duration of such storage sometimes extends for a long period. Therefore, the manufacture of paints that will cure by the action of moisture necessitates the filling of their containers with a dry nitrogen gas. In addition, once the container is opened, aerial moisture will get in to cause curing of the surface of the paint or an increase in its viscosity. Paint that has undergone such changes is no longer suitable for use.